SUMMARY Mitochondrial calcium (mCa2+) overload is a central event in myocardial infarction (MI) and heart failure (HF), causing metabolic derangement, mitochondrial permeability transition pore (MPTP) activation, genetic reprograming and loss of cells due to necrosis. The Mitochondrial Calcium Uniporter Channel (mtCU) is the primary mechanism for mCa2+ uptake, located at the inner mitochondrial membrane (IMM), and physiologically is required for activation of mitochondrial energetic pathways to support contractility during stress (fight-or-flight response). The mtCU is a multiprotein high-MW channel, ~400-800 kD in size, containing pore-forming, scaffold and regulatory components. Given the critical roles in metabolism and cell death it is of great scientific interest to understand the mechanisms regulating mCa2+ uptake. We recently reported that conditional genetic ablation of MCU, the pore forming subunit of the mtCU, is cardioprotective in an in vivo model of ischemia- reperfusion (IR) injury, providing evidence of translational potential. Recently a gene paralog of MCU, CCDC109b (MCUB), was identified as a component of the uniporter and is theorized to negatively regulate mCa2+ uptake. However, no genetic or in vivo studies have investigated this gene and it's mechanism of action remains unknown. Given the therapeutic value of deciphering uniporter channel regulation, this proposal will examine the molecular function of MCUB, and its contribution to mCa2+ dynamics in cardiac physiology and disease using in vitro and in vivo genetic gain- and loss-of-function approaches. We hypothesize that MCUB is a stress-responsive regulator of mitochondrial calcium uptake by modulating the composition (members and stoichiometry) of the mtCU and that therapeutic manipulation of MCUB will reduce pathogenic mCa2+ uptake, as occurs in IR injury and heart failure (HF).